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Here I would like to discuss the difference between and the significance of Optical Resolution versus Interpolated Resolution. I've opened up an image of an O that I've scanned with my scanner at a resolution of 1200 pixels per inch. I chose 1200 pixels per inch for two reasons. One, it gives me enough detail that I can get a really sharp consistent edge as we'll see, but I also know that this is the optical also known as hardware resolution of the scanner. What that means is that the scanner is actually set up to actually physically capture pixels at one- 1200th of an inch on the side.
So this is going to capture 1200 pixels in every inch horizontally and 1200 pixels in every inch vertically. The reason why this is important will become clear in just a moment. So this is a 1200 pixel print scan and I'm going to zoom in on this so we can take a look at these two edges right here and you'll notice there is a real consistency to the placement of pixels. There is a little missteps every once in a while, but in general this is very, very consistent edge and the same thing on the inside. What I'm going to do now is I'm going to make a duplicate copy of this image and I'm just going to label this 1700 and we're going to convert this image from a 1200 pixel per inch image to a 1700 pixel per inch image in Photoshop.
Notice a couple of things, one is, by default and I'm leaving it that way I'm re-sampling this image, watch what happens when I resample this image from the 1200. The original pixel dimension is 540x442, when I go up to 1700 the number of pixels goes from 765 to 626. We're adding pixels, we're adding them by interpolating, by using the pixels that are already there to create more pixels, this is what interpolation does. I'm going to click OK and then I'm going to reposition this so we can see these pixels in the same place that we see them here.
Now let's compare and in fact contrast these two different edges. Look at this very, very nice smooth consistent edge and look what's happened to this edge, do you see how much rougher it is? It's even worse on the inside, and that's just one step of interpolation from 1200 to 1700, we haven't rotated or skewed or done anything. And what this demonstrates is that if you interpolate your image either during the scan or in the post scan you're going to lose edge quality, the quality of your image is going to go down, because new pixels are created from old pixels and they're never as good as the original ones.
This is particularly important if in the very beginning phases you decide oh I want this line art image, in this case this O to be converted into vectors. When we do a conversion of vectors the vectorization software loops the consistency of this edge and the smoother and more consistent the edge is the higher the quality of the vector the smoother the vector is going to be. But the same thing is true for continuous tone images, the more you resample them the lower the quality they're going to be, they're going to lose sharpness, they're going to lose definition. So our general rule of thumb is we want to avoid interpolation whenever possible, and what this involves is scanning, using the optical resolution of the scanner whenever we can.
And in the case of vectors and we'll return to this later wait until you've converted your images into vectors before you do any geometric manipulation of them. Because remember as long as you've got pixels and you're doing geometric manipulation the quality of your edges are going to be lost. There are some cases where you actually can embrace the enemy where you've got damaged images, where you can actually use this interpolation to your benefit to kind of smooth out some of the damages, but as a general rule we want to avoid interpolation whenever possible and as far as scanning goes that means setting the proper resolution to begin with.
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